Iron base sintered alloy having highly densified and hardened surface, and producing method thereof

ABSTRACT

An iron base sintered alloy having highly densified and hardened surface and a producing method thereof are provided. The iron base sintered alloy includes an iron base sintered alloy surface layer portion having a thickness in a range of 1 μm to 2 mm from a surface of the iron base sintered alloy and containing K and/or Na in an average concentration of 0.0001 to 1 mass %, or having an average concentration of P higher than a concentration of P in an iron base sintered alloy inner portion located inside the iron base sintered alloy surface layer portion and containing K and/or Na in an average concentration of 0.0001 to 1 mass %.

CROSS-REFERENCE TO PRIOR APPLICATION

This is a U.S. National Phase application under 35 U.S.C. §371 of International Patent Application No. PCT/JP2004/008880 filed Jun. 24, 2004, and claims the benefit of Japanese Patent Application Nos. 2003-183802 filed Jun. 27, 2003 and 2003-183803 filed Jun. 27, 2003, all of which are incorporated by reference herein. The International Application was published in Japanese on Jan. 6, 2005 as WO 2005/001150 Al under PCT Article 21(2).

BACKGROUND

1. Technical Field

The present invention relates to an iron base sintered alloy (hereinafter, referred to as “iron base sintered alloy surface layer portion”) having a thickness in a range of 1 μm to 2 mm from a surface of the iron base sintered alloy; and an inner portion of the iron base sintered alloy (hereinafter, referred to as “iron base sintered alloy inner portion”) located inside the iron base sintered alloy surface layer portion, the surface layer portion of which contains one or two kinds of potassium (K) and sodium (Na), and producing method thereof. The iron base sintered alloy is available particularly for mechanical parts requiring surface hardness and wear resistance, for example, sprockets, gears, rotors, inner races, outer races etc. Furthermore, since having highly densified surfaces, the iron base sintered alloy of the invention is available for mechanical parts made of the iron base sintered alloy required plating on the surface.

2. Related Art

In general, it is well-known that an iron base sintered alloy has been used for a member for various mechanical parts, and the iron base sintered alloy is produced by sintering a powder compact, which is fabricated by mixing a metallic salt of a high fatty acid such as zinc stearate, calcium stearate, lithium stearate etc. with the iron base alloy powder as a lubricant and then filling the mixture into a cavity of a pressing die for press-shaping.

However, the powder compact obtained by pressing the powder mixed with the conventional metallic salt of a high fatty acid, filled in the cavity of the pressing die, contains a great amount of lubricant, thereby the powder compact cannot have a high density. In addition, the lubricant remaining in the iron base sintered alloy deteriorates the mechanical strength of the iron base sintered alloy produced by sintering the powder compact obtained by the above method.

On the other hand, when a small amount of lubricant is added to the iron base alloy powder in order to fabricate the powder compact containing a small amount of lubricant, and the iron base sintered alloy having excellent mechanical strength is produced by sintering the powder compact containing a small amount of lubricant, the shortage of the amount of lubricant leads to the lack of lubricity, thereby the pressed powder compact is hard to remove from the pressing die, and the yield decreases due to inferior powder compact.

In this case, in order to decrease the difficulty in removing the powder compact from the pressing die due to the lack of the lubricant, a high fatty acid lubricant including the metallic salt of a high fatty acid (for example, lithium stearate, calcium stearate, or zinc stearate) suspended on a solvent is applied to an inner surface of the cavity of the pressing die, and the die is heated up to 100° C. or more to form a coat of the metallic acid of a high fatty acid by evaporating the solvent. After that, the iron base sintered alloy powder containing a smaller amount of the lubricant than usual is filled in the cavity, on the inner surface of which the coat of the metallic acid of a high fatty acid is formed, and then pressed at a pressure of 600 MPa or more so as to fabricate the powder compact. Then, an iron salt coat of a high fatty acid such as a monomolecular coat of iron stearate is generated on the surface of the powder compact, thereby a friction between the powder compact and the die is reduced, and a pressure requiring for the removal of the powder compact decreases, thereby the powder compact can be removed from the die easily. Furthermore, since the powder compact is pressed at a high pressure of 600 MPa or more, powder compact having high density can be produced (See Patent No. 3309970).

According to the above method, powder compact having a small amount of the lubricant and a high density can be obtained, and a force required to remove the powder compact from the press mold can be decreased since the removal of the powder compact is facilitated. Furthermore, the powder compact rarely breaks during the removal. As a result, excellent powder compact can be fabricated efficiently and excellent iron base sintered alloy can be produced by sintering the powder compact.

However, the lubricant coat obtained by applying the high fatty acid lubricant including the solid metallic salt of a solid-state high fatty acid such as zinc stearate, calcium state, lithium stearate or the like, which are solid powder, suspended in water to the inner surface of the cavity of the pressing die is formed as a solid powder coat on the inner surface of the cavity, thereby the coat cannot be densified on the inner surface of the cavity of the pressing die and adhered to the inner surface of the cavity sufficiently. As a result, there is a problem in that a stable lubricant coat is hard to form.

SUMMARY

Therefore, the present inventors have studied the way of forming denser lubricant coat on the inner surface of the cavity of the pressing die from the above viewpoints and, finally, found out the followings;

(a) a solution solving a soluble potassium phosphate metallic salt and/or sodium phosphate metallic salt in a solvent is made. The solution is adopted as a lubricant and applied to an inner surface of a cavity of a pressing die that is heated higher than a temperature at which the solvent of the lubricant is evaporated. Then, since the pressing die is heated at such a high temperature that the solvent can be evaporated, the solvent contained in the lubricant is evaporated, and the potassium sulfate salt and/or sodium sulfate salt are/is crystallized onto the inner surface of the cavity of the pressing die, so that a crystallized coat of potassium sulfate salt and/or sodium sulfate salt is formed on the inner surface of the cavity of the pressing die. The crystallized coat of potassium sulfate salt and/or sodium sulfate salt formed on the inner surface of the cavity of the pressing die can form an extremely dense lubricant coat and has excellent adhesion as well. As a result, the coat is hardly exfoliated during working.

(b) Raw powder, iron base sintered alloy powder, is filled into the cavity of the pressing die, on the inner surface of which the crystallized coat of potassium sulfate salt and/or sodium sulfate salt is formed, and the filled powder is pressed to form a powder compact. Since the crystallized coat of potassium sulfate salt and/or sodium sulfate salt is formed on the inner surface of the cavity of the pressing die, the powder compact can be drawn out easily from the pressing die. Further, a surface of the obtained powder compact contains potassium (hereinafter, referred to as “K”) and/or sodium (hereinafter, referred to as “Na”). When the powder compact containing K and/or Na on the surface thereof is sintered, K and/or Na reduce(s) an iron oxide coat to activate a surface of the iron base alloy powder. Thereby, a dense-surfaced iron base sintered alloy having extremely small open pores (communicating from the surface of the sintered alloy to the inside) on the surface of a sintered body is obtained. The dense-surfaced iron base sintered alloy is increased in surface hardness and thus in surface wear resistance. As such, the iron base sintered alloy is used as a member for mechanical parts such as sprockets, gears, rotors, inner races, outer races or the like which require wear resistance. In addition, highly densified surface makes the iron base sintered alloy available for a member such as mechanical part and magnetic part made of the iron base alloy, which need plating on the surface.

(c) The iron base sintered alloy having highly densified and hardened surface obtained in this way contains K and/or Na in a range of 0.0001 to 1 mass % in an iron base sintered alloy surface layer portion.

(d) Concentrations of K and Na in the iron base sintered alloy surface layer portion are generally detected or not according to depth and surface positions from the surface of the iron base sintered alloy when subjected to high precision analysis, thus showing great deviation. For this reason, it is preferable that the concentrations of K and Na in the iron base sintered alloy surface layer portion are obtained as an average value.

Alternatively,

(e) a solution solving a soluble potassium phosphate metallic salt and/or sodium phosphate metallic salt in a solvent is made. The solution is adopted as a lubricant and applied to an inner surface of a cavity of a pressing die that is heated higher than a temperature at which the solvent of the lubricant is evaporated. Then, since the pressing die is heated at such a high temperature that the solvent can be evaporated, the solvent contained in the lubricant is evaporated, and the potassium phosphate metallic salt and/or sodium phosphate metallic salt are/is crystallized onto the inner surface of the cavity of the pressing die, so that a crystallized coat of potassium phosphate metallic salt and/or sodium phosphate metallic salt is formed on the inner surface of the cavity of the pressing die. The crystallized coat of potassium phosphate metallic salt and/or sodium phosphate metallic salt which is formed on the inner surface of the cavity of the pressing die can form an extremely dense lubricant coat and has excellent adhesion as well. As a result, the coat is hardly exfoliated during working.

(f) Raw powder, iron base sintered alloy powder, is filled into the cavity of the pressing die on the inner surface of which the crystallized coat of potassium phosphate metallic salt and/or sodium phosphate metallic salt is formed, and the filled powder is pressed to form a powder compact. Since the crystallized coat of potassium phosphate metallic salt and/or sodium phosphate metallic salt is formed on the inner surface of the cavity of the pressing die, the powder compact is possible to easily draw out from the pressing die. Further, a surface of the obtained powder compact has a high concentration of phosphor (hereinafter, referred to as “P”) and contains potassium (hereinafter, referred to as “K”) and/or sodium (hereinafter, referred to as “Na”). When the powder compact having the high concentration of P on the surface thereof and containing K and/or Na is sintered, the surface having the concentration of P is easily subjected to liquid and a phase sintering, and containing K and/or Na causes an iron oxide coat to be reduced to activate a surface of the iron base alloy powder. Thereby, a dense surface iron base sintered alloy where open pores (communicating from the surface of the sintered alloy to the inside) are extremely small on the surface of a sintered body is obtained. The dense surface iron base sintered alloy is increased in surface hardness and thus in surface wear resistance. As such, the iron base sintered alloy is available as a material for mechanical parts such as sprockets, gears, rotors, inner races, outer races or the like which require wear resistance. In addition, highly densified surface makes the iron base sintered alloy available for mechanical part and magnetic part made of the iron base alloy, which need plating on the surface.

(g) In the iron base sintered alloy having highly densified and hardened surface obtained in this way, a depth portion (hereinafter, referred to as “iron base sintered alloy surface layer portion) located in a range of 1 μm to 2 mm (preferably, a range of 10 μm to 0.5 mm) from the surface of the iron base sintered alloy contains P in an average concentration of 0.01 to 1 mass % (more preferably, 0.02 to 0.5 mass %) higher than a concentration of P contained in a portion (hereinafter, referred to as “iron base sintered alloy inner portion) located inside the iron base sintered alloy surface layer portion. Further, the iron base sintered alloy surface layer portion contains K and/or Na within a range of 0.0001 to 1 mass %.

(h) Concentrations of P, K and Na in the iron base sintered alloy surface layer portion which corresponds to the depth portion located in a range of 1 μm to 2 mm (preferably, a range of 10 μm to 0.5 mm) from the surface of the iron base sintered alloy are generally detected or not according to depth and surface positions from the surface of the iron base sintered alloy when being subjected to high density analysis, thus showing great deviation. For this reason, it is preferable that the concentrations of P, K and Na in the iron base sintered alloy surface layer portion are obtained as an average value.

The present invention is made on the basis of this knowledge, and characterized by:

(1) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy surface layer portion contains K;

(2) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy surface layer portion contains Na;

(3) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy surface layer portion contains K and Na;

(4) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy surface layer portion contains K in an average concentration of a range of 0.0001 to 1 mass %;

(5) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy surface layer portion contains Na in an average concentration of 0.0001 to 1 mass %; and

(6) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy surface layer portion contains K and Na in an average concentration of 0.0001 to 1 mass % in total.

As a material for mechanical parts such as sprockets, gears, rotors, inner races, outer races etc. which require surface wear resistance, it is generally known to use the iron base sintered alloy which contains C of 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu of 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S of 0.1 to 3 mass % in total if necessary, and a balance of Fe and unavoidable impurities. In the iron base sintered alloy of the invention having highly densified and hardened surface, the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % (including 0 mass %), one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total if necessary, and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % (including 0 mass %), one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total if necessary, one or two of K and Na: 0.0001 to 1 mass % in an average concentration in total, and balance: Fe and unavoidable impurities.

Therefore, the invention is characterized by:

(7) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, K: 0.0001 to 1 mass % in the average concentration, and balance: Fe and unavoidable impurities;

(8) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, Na: 0.0001 to 1 mass % in the average concentration, and balance: Fe and unavoidable impurities;

(9) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, K and Na: 0.0001 to 1 mass % in the average concentration in total, and balance: Fe and unavoidable impurities;

(10) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, K: 0.0001 to 1 mass % in the average concentration, and balance: Fe and unavoidable impurities;

(11) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, Na: 0.0001 to 1 mass % in the average concentration, and balance: Fe and unavoidable impurities; and

(12) an iron base sintered alloy having highly densified and hardened surface, including: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, in which the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, K and Na : 0.0001 to 1 mass % in the average concentration in total, and balance: Fe and unavoidable impurities.

In the iron base sintered alloys (1) to (12) having highly densified and hardened surface of the invention, the iron base sintered alloy surface layer portion has a dense surface and has extremely small pores, and particularly open pores (communicating from the surface of the sintered alloy to the inside) on the surface thereof, thus having open porosity of 5% or less. Therefore, the invention is characterized by

(13) an iron base sintered alloy having highly densified and hardened surface, according to any one of the iron base sintered alloys (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11) and (12), in which the iron base sintered alloy surface layer portion has open porosity of 5% or less.

The open porosity may be preferably small, more preferably 2% or less, and still more preferably 1% or less.

In the invention, the iron base sintered alloy having highly densified and hardened surface, according to any one of the iron base sintered alloys (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11) and (12) may be used as a material for various mechanical parts, and preferably as a material for parts such as sprockets, gears, rotors, inner races, outer races etc. which require strength and undergo surface wear with ease. Therefore, the invention is characterized by:

(14) a mechanical part made of an iron base sintered alloy having highly densified and hardened surface, according to any one of the iron base sintered alloys (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11) and (12);

(15) a mechanical part according to the mechanical part (14), which is a sprocket;

(16) a mechanical part according to the mechanical part (14), which is a gear;

(17) a mechanical part according to the mechanical part (14), which is a rotor;

(18) a mechanical part according to the mechanical part (14), which is an inner race; and

(19) a mechanical part according to the mechanical part (14), which is an outer race.

Further, the invention is characterized by:

(20) an iron base sintered alloy having highly densified and hardened surface, in which an iron base sintered alloy surface layer portion having a thickness in a range of 1 μm to 2 mm from a surface of the iron base sintered alloy has an average concentration of P higher than a concentration of P of an iron base sintered alloy inner portion located inside the iron base sintered alloy surface layer portion, and the iron base sintered alloy surface layer portion contains K;

(21) an iron base sintered alloy having highly densified and hardened surface, in which an iron base sintered alloy surface layer portion has an average concentration of P higher than a concentration of P of an iron base sintered alloy inner portion, and the iron base sintered alloy surface layer portion contains Na;

(22) an iron base sintered alloy having highly densified and hardened surface, in which an iron base sintered alloy surface layer portion has an average concentration of P higher than a concentration of P of an iron base sintered alloy inner portion, and the iron base sintered alloy surface layer portion contains K and Na;

(23) an iron base sintered alloy having highly densified and hardened surface, in which the average concentration of P of the iron base sintered alloy surface layer portion is higher than the concentration of P of the iron base sintered alloy inner portion by 0.01 to 1 mass %, and the iron base sintered alloy surface layer portion contains K in an average concentration of 0.0001 to 1 mass %;

(24) an iron base sintered alloy having highly densified and hardened surface, in which the average concentration of P of the iron base sintered alloy surface layer portion is higher than the concentration of P of the iron base sintered alloy inner portion by 0.01 to 1 mass %, and the iron base sintered alloy surface layer portion contains Na in an average concentration of 0.0001 to 1 mass %; and

(25) an iron base sintered alloy having highly densified and hardened surface, in which the average concentration of P of the iron base sintered alloy surface layer portion is higher than the concentration of P of the iron base sintered alloy inner portion by 0.01 to 1 mass %, and the iron base sintered alloy surface layer portion contains K and Na in an average concentration of 0.0001 to 1 mass % in total.

As a material for mechanical parts such as sprockets, gears, rotors, inner races, outer races etc. which require surface wear resistance, it is generally known to use the iron base sintered alloy which contains C of 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu of 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S of 0.1 to 3 mass % in total if necessary, and a balance of Fe and unavoidable impurities. It is known that the iron base sintered alloy contains 0.05 mass % (including 0 mass %) of P as an unavoidable impurity. In the iron base sintered alloy of the invention having highly densified and hardened surface, the iron base sintered alloy inner portion is made up of an iron base alloy sintered body containing C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % (including 0 mass %), one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total if necessary, and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion is made up of an iron base alloy sintered body containing C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % (including 0 mass %), one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total if necessary, P: an average concentration higher than a concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass %, one or two of K and Na: 0.0001 to 1 mass % in an average concentration in total, and balance: Fe and unavoidable impurities.

Therefore, the invention is characterized by:

(26) an iron base sintered alloy having highly densified and hardened surface, in which the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains K in an average concentration of 0.0001 to 1 mass %;

(27) an iron base sintered alloy having highly densified and hardened surface, in which the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains Na in an average concentration of 0.0001 to 1 mass %;

(28) an iron base sintered alloy having highly densified and hardened surface, in which the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains K and Na in an average concentration of 0.0001 to 1 mass % in total;

(29) an iron base sintered alloy having highly densified and hardened surface, in which the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains K in an average concentration of 0.0001 to 1 mass %;

(30) an iron base sintered alloy having highly densified and hardened surface, in which the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains Na in an average concentration of 0.0001 to 1 mass %; and

(31) an iron base sintered alloy having highly densified and hardened surface, in which the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains K and Na in an average concentration of 0.0001 to 1 mass % in total.

In the iron base sintered alloys (20) to (31) of the invention, the iron base sintered alloy surface layer portion has a dense surface and has extremely small pores, and particularly open pores (communicating from the surface of the sintered alloy to the inside) on the surface thereof, because the surface portion of the powder compact is reduced to undergo liquid and a phase sintering again, thus having open porosity of 5% or less. Therefore, the invention is characterized by

(32) an iron base sintered alloy having highly densified and hardened surface, according to any one of the iron base sintered alloys (20), (21), (22), (23), (24), (25), (26), (27), (28), (29), (30) and (31), in which the iron base sintered alloy surface layer portion has open porosity of 5% or less.

The open porosity may be preferably small, more preferably 2% or less, and still more preferably 1% or less.

In the invention, the iron base sintered alloy having highly densified and hardened surface, according to any one of the iron base sintered alloys (20), (21), (22), (23), (24), (25), (26), (27), (28), (29), (30), (31) and (32) may be used as a material for various mechanical parts, and preferably as a material for parts such as sprockets, gears, rotors, inner races, outer races etc. which require strength and undergo surface wear with ease. Therefore, the invention is characterized by:

(33) a mechanical part made of the iron base sintered alloy according to any one of the iron base sintered alloys (20), (21), (22), (23), (24), (25), (26), (27), (28), (29), (30), (31) and (32).

(34) a mechanical part according to the mechanical part (33), which is a sprocket;

(35) a mechanical part according to the mechanical part (33), which is a gear;

(36) a mechanical part according to the mechanical part (33), which is a rotor;

(37) a mechanical part according to the mechanical part (33), which is an inner race; and

(38) a mechanical part according to the mechanical part (33), which is an outer race.

In the invention, the term “iron base sintered alloy surface layer portion” refers to a portion having a thickness of a range of 1 μm to 2 mm from the surface of the iron base sintered alloy. When the thickness of the iron base sintered alloy surface layer portion is less than 1 μm, it is not favorable because the effect on the improvement of surface and surface hardness is not remarkable. In contrast, when the thickness of the iron base sintered alloy surface layer portion is more than 2 mm, this is also unfavorable because liquid sintering and a phase sintering proceed up to the inner portion of the sintered alloy, and thus dimension precision is lowered, and furthermore it is easy to generate a crack during quenching.

Further, in the invention, the term “iron base sintered alloy inner portion” is a portion located inside the iron base sintered alloy surface layer portion having the thickness of 1 μm to 2 mm from the surface of the iron base sintered alloy.

In the iron base sintered alloy surface having highly densified and hardened surface according to the invention, restricting an average concentration of one or two of K and Na to 0.0001 mass % or more in total in the iron base sintered alloy surface layer portion is because an effect of reducing and activating a surface oxide coat of iron base sintered alloy raw powder when the average concentration of these ingredients is less than 0.0001 mass %, and thus it is impossible to obtain a dense surface layer portion. Meanwhile, in the iron base sintered alloy surface layer portion, increasing the average concentration of these ingredients higher than 1 mass % is not allowed on the ordinary sintering condition, and particularly fails to increase surface densification. Thus, in the iron base sintered alloy surface having highly densified and hardened surface according to the invention, the average concentration of one or two of K and Na contained in the iron base sintered alloy surface layer portion is restricted to a range of 0.0001 to 1 mass % in total. In the iron base sintered alloy surface having highly densified and hardened surface according to the invention, restricting the average concentration value of one or two of K and Na contained in the iron base sintered alloy surface layer portion to a range of 0.02 to 0.5 mass % in total is easy to produce the iron base sintered alloy and is more preferable from the viewpoint of production efficiency.

In the iron base sintered alloy surface exhibiting excellent surface densification, surface hardness and corrosion resistance according to the invention, restricting an average concentration value of P in the iron base sintered alloy surface layer portion to be higher than a concentration of P in the iron base sintered alloy inner portion by 0.01 mass % is because a content of P contained in the ordinary iron base sintered alloy inner portion is extremely small when a difference between the average concentration value of P in the iron base sintered alloy surface layer portion and the concentration of P in the iron base sintered alloy inner portion is less than 0.01 mass %, and thus there is little difference in P content between the iron base sintered alloy surface layer portion and the iron base sintered alloy inner portion, and the P content of the iron base sintered alloy surface layer portion is small. For this reason, the effects on the liquid sintering and a phase sintering are not generated. Thus, the surface of the iron base sintered alloy does not become dense, so that it is impossible to obtain high surface hardness.

Meanwhile, restricting the difference between the P concentration of the iron base sintered alloy and the average value of the P concentration in the iron base sintered alloy inner portion to 1 mass % or less is based on the fact that increasing the P concentration of the iron base sintered alloy surface layer portion enough to exceed 1 mass % is not allowed on the ordinary sintering condition, and that, even when the P concentration of the iron base sintered alloy surface layer portion is increased such that the difference between the average value of the P concentration in the iron base sintered alloy surface layer portion and the P concentration in the iron base sintered alloy inner portion exceeds 1 mass %, the surface densification and surface hardness are not increased. When the average value of the P concentration in the iron base sintered alloy surface layer portion becomes higher than the P concentration in the iron base sintered alloy inner portion by a range of 0.02 to 0.5 mass %, it is easy to produce the iron base sintered alloy. Thus, this is still more preferable from the viewpoint of production efficiency. Further, in the iron base sintered alloy having highly densified and hardened surface according to the invention, restricting the average concentration of one or two of K and Na to 0.0001 mass % in total in the iron base sintered alloy surface layer portion is based on the fact that, when the average concentration of these ingredients is less than 0.0001 mass %, the effect reducing and activating the surface oxide coat of the iron base sintered alloy raw material powder is insufficient to fail to obtain the dense surface layer portion. On the other hand, when the average concentration of these ingredients is increased enough to exceed 1 mass % in the iron base sintered alloy surface layer portion, this is not allowed on the ordinary sintering condition, and particularly the surface densification is no longer increased. Therefore, in the iron base sintered alloy surface having highly densified and hardened surface according to the invention, the average concentration of one or two of K and Na contained in the iron base sintered alloy surface layer portion is restricted to a range of 0.0001 to 1 mass %. In the iron base sintered alloy surface having highly densified and hardened surface according to the invention, when the average concentration value of one or two of K and Na contained in the iron base sintered alloy surface layer portion is restricted to a range of 0.02 to 0.5 mass %, it is easy to produce the iron base sintered alloy. This is still more preferable from the viewpoint of production efficiency.

The iron base sintered alloy (1), (4), (7) or (10) having highly densified and hardened surface is produced by: forming a powder compact containing K on a surface attached with a potassium sulfate salt by forming a coat of the potassium sulfate salt consisting of one or at least two of potassium sulfate, potassium sulfite, potassium thiosulfate, potassium dodecyl sulfate, potassium dodecylbenzene sulfate, food blue No. 1 and potassium ascorbate sulfate ester, each of which acts as a lubricant, on an inner surface of a cavity of a pressing die, by filling raw powder of the iron base sintered alloy into the cavity of the pressing die, and by pressing the filled raw powder; and sintering the formed powder compact at a temperature of 1000 to 1300° C.

In order to form the coat of the potassium sulfate salt on the inner surface of the cavity of the pressing die, a solution solving the potassium sulfate salt in a solvent (e.g. water) is applied to the inner surface of the cavity of the heated pressing die, the solvent (e.g. water) is evaporated, and the potassium sulfate salt as the lubricant is crystallized. At this time, because a temperature at which the solvent (e.g. water) is evaporated will do as the temperature of heating the pressing die, the temperature of heating the pressing die may be more than 100° C.

The iron base sintered alloy (2), (5), (8) or (11) having highly densified and hardened surface is produced by: forming a powder compact containing Na on a surface attached with a sodium sulfate salt by forming a coat of the sodium sulfate salt consisting of one or at least two of sodium sulfate, sodium sulfite, sodium thiosulfate, sodium dodecyl sulfate, sodium dodecylbenzene sulfate, food blue No. 5 and sodium ascorbate sulfate ester, each of which acts as a lubricant, on an inner surface of a cavity of a pressing die, by filling raw powder of the iron base sintered alloy into the cavity of the pressing die, and by pressing the filled raw powder; and sintering the formed powder compact at a temperature of 1000 to 1300° C.

In order to form the coat of the sodium sulfate salt on the inner surface of the cavity of the pressing die, a solution solving the sodium sulfate salt in a solvent (e.g. water) is applied to the inner surface of the cavity of the heated pressing die, the solvent (e.g. water) is evaporated, and the sodium sulfate salt as the lubricant is crystallized. At this time, because a temperature at which the solvent (e.g. water) is evaporated will do as the temperature of heating the pressing die, the temperature of heating the pressing die may be more than 100° C.

The iron base sintered alloy (3), (6), (9) or (12) having highly densified and hardened surface is produced by: forming a powder compact containing K and Na on a surface attached with a potassium sulfate salt and a sodium sulfate salt in a high concentration in total by forming a mixed coat of the potassium sulfate salt and the sodium sulfate salt which acts as a lubricant on an inner surface of a cavity of a pressing die, by filling raw powder of the iron base sintered alloy into the cavity of the pressing die, and by pressing the filled raw powder; and sintering the formed powder compact at a temperature of 1000 to 1300° C.

In order to form the coat of the mixed salt of the potassium sulfate salt and sodium sulfate salt on the inner surface of the cavity of the pressing die, a solution solving the mixed salt of the potassium sulfate salt and sodium sulfate salt in a solvent (e.g. water) is applied to the inner surface of the cavity of the heated pressing die, the solvent (e.g. water) is evaporated, and the mixed salt of the potassium sulfate salt and sodium sulfate salt as the lubricant is crystallized. At this time, because a temperature at which the solvent (e.g. water) is evaporated will do as the temperature of heating the pressing die, the temperature of heating the pressing die may be more than 100° C.

The mechanical part made of the iron base sintered alloy (4) having highly densified and hardened surface and the iron base sintered alloys (14) to (19) having highly densified and hardened surface may be produced by the same method as that for producing the iron base sintered alloy having highly densified and hardened surface according to the invention.

The raw powder used when the iron base sintered alloy having highly densified and hardened surface according to the invention is produced makes use of a mixed powder where C powder is added to adjust a content of C with respect to a known iron base alloy powder having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, and balance: Fe and unavoidable impurities, which is known as raw powder used for producing mechanical parts requiring surface wear resistance such as sprockets, gears, rotors, inner races, outer races and so on.

The iron base sintered alloy (20), (23), (26) or (29) having highly densified and hardened surface is produced by: forming a powder compact where a surface to which a potassium phosphate metallic salt is attached has a concentration of P higher than that of an inner portion thereof and which contains K on the surface thereof by forming a coat of the potassium phosphate metallic salt consisting of one or at least two of dipotassium hydrogen phosphate, tripotassium phosphate, polypotassium phosphate, potassium riboflavin phosphate, each of which acts as a lubricant, on an inner surface of a cavity of a pressing die, by filling raw powder of the iron base sintered alloy into the cavity of the pressing die, and by pressing the filled raw powder; and sintering the formed powder compact at a temperature of 1000 to 1300° C.

In order to form the coat of the potassium phosphate metallic salt on the inner surface of the cavity of the pressing die, a solution solving the potassium phosphate metallic salt in a solvent (e.g. water) is applied to the inner surface of the cavity of the heated pressing die, the solvent (e.g. water) is evaporated, and the potassium phosphate metallic salt as the lubricant is crystallized. At this time, because a temperature at which the solvent (e.g. water) is evaporated will do as the temperature of heating the pressing die, the temperature of heating the pressing die may be more than 100° C.

The iron base sintered alloy (21), (24), (27) or (30) having highly densified and hardened surface is produced by: forming a powder compact where a surface to which a sodium phosphate metallic salt is attached has a concentration of P higher than that of an inner portion thereof and which contains Na on the surface thereof by forming a coat of the sodium phosphate metallic salt consisting of one or at least two of disodium hydrogen phosphate, trisodium phosphate, polysodium phosphate, sodium riboflavin phosphate, each of which acts as a lubricant, on an inner surface of a cavity of a pressing die, by filling raw powder of the iron base sintered alloy into the cavity of the pressing die, and by pressing the filled raw powder; and sintering the formed powder compact at a temperature of 1000 to 1300° C.

In order to form the coat of the sodium phosphate metallic salt on the inner surface of the cavity of the pressing die, a solution solving the sodium phosphate metallic salt in a solvent (e.g. water) is applied to the inner surface of the cavity of the heated pressing die, the solvent (e.g. water) is evaporated, and the sodium phosphate metallic salt as the lubricant is crystallized. At this time, because a temperature at which the solvent (e.g. water) is evaporated will do as the temperature of heating the pressing die, the temperature of heating the pressing die may be more than 100° C.

The iron base sintered alloy (22), (25), (28) or (31) having highly densified and hardened surface is produced by: forming a powder compact where a surface to which a mixed metallic salt of a potassium phosphate metallic salt and a sodium phosphate metallic salt is attached has a concentration of P higher than that of an inner portion thereof and which contains K and Na on the surface thereof by forming a coat of the mixed metallic salt of the potassium phosphate metallic salt and the sodium phosphate metallic salt which acts as a lubricant, on an inner surface of a cavity of a pressing die, by filling raw powder of the iron base sintered alloy into the cavity of the pressing die, and by pressing the filled raw powder; and sintering the formed powder compact at a temperature of 1000 to 1300° C.

In order to form the mixed metallic salt of the potassium phosphate metallic salt and the sodium phosphate metallic salt on the inner surface of the cavity of the pressing die, a solution solving the mixed metallic salt of the potassium phosphate metallic salt and the sodium phosphate metallic salt in a solvent (e.g. water) is applied to the inner surface of the cavity of the heated pressing die, the solvent (e.g. water) is evaporated, and the mixed metallic salt of the potassium phosphate metallic salt and the sodium phosphate metallic salt as the lubricant is crystallized. At this time, because a temperature at which the solvent (e.g. water) is evaporated will do as the temperature of heating the pressing die, the temperature of heating the pressing die may be more than 100° C.

The mechanical part made of the iron base sintered alloy (32) having highly densified and hardened surface and the iron base sintered alloys (33) to (38) having highly densified and hardened surface may be produced by the same method as that for producing the iron base sintered alloy having highly densified and hardened surface according to the invention.

The raw powder used when the iron base sintered alloy having highly densified and hardened surface according to the invention is produced makes use of a mixed powder where C powder is added to adjust a content of C with respect to a known iron base alloy powder having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % (including 0 mass %), one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, and balance: Fe and unavoidable impurities, which is known as raw powder used for producing mechanical parts requiring surface wear resistance such as sprockets, gears, rotors, inner races, outer races and so on.

As set forth above, the iron base sintered alloy of the invention has high surface hardness and low open porosity as compared with the conventional iron base sintered alloy having the same ingredient composition, so that it is easy to carry out surface treatment such as plating etc. As a result, the iron base sintered alloy of the invention can be used as a member for various mechanical parts requiring wear resistance, and can make a great contribution to development of mechanical industry.

DESCRIPTION OF THE EMBODIMENTS Embodiment 1

Pressing dies were prepared, each of which had a cavity of a diameter of 11 mm and a height of 40 mm, and inner surfaces of the cavities of which could be heated at a predetermined temperature. Further, as raw powders, Fe-1.5% Mo iron base alloy powder having an average particle size of 90 μm and C powder of an average particle size of 20 μm were prepared, and blended and mixed to be Fe-1.5% Mo-0.3% C with a V-shaped blender. Thereby, mixed powders were obtained.

Further, as potassium sulfate salts, there were prepared potassium sulfate, potassium sulfite, potassium thiosulfate, potassium dodecyl sulfate, potassium dodecylbenzene sulfate, food blue No. 1 and potassium ascorbate sulfate ester. Further, as sodium sulfate salts, there were prepared sodium sulfate, sodium sulfite, sodium thiosulfate, sodium dodecyl sulfate, sodium dodecylbenzene sulfate, food blue No. 5 and sodium ascorbate sulfate ester.

These salts were solved into a solvent, water, in a different concentration, and thereby lubricants A to U and a to f denoted in Table 1-1 were obtained. TABLE 1-1 Composition (mass %) Type Additives Balance Lubricant A potassium sulfate: 1 water B potassium sulfite: 1 C potassium thiosulfate: 1 D potassium dodecyl sulfate: 1 E food blue No. 1: 1 F potassium dodecylbenzene sulfate: 1 G potassium ascorbate sulfate ester: 1 a potassium sulfate: 0.001 b potassium sulfate: 3 H sodium sulfate: 1 I sodium sulfite: 1 J sodium thiosulfate: 1 K sodium dodecyl sulfate: 1 L food blue No. 5: 1 M sodium dodecylbenzene sulfate: 1 N sodium ascorbate sulfate ester: 1 c sodium sulfate: 0.001 d sodium sulfate: 3 O potassium sulfate: 0.5, sodium sulfate: 0.5 P potassium sulfite: 0.5, sodium sulfite: 0.5 Q potassium thiosulfate: 0.5, sodium thiosulfate: 0.5 R potassium dodecyl sulfate: 0.5, sodium dodecyl sulfate: 0.5 S food blue No. 1: 0.5, food blue No. 5: 0.5 T potassium dodecylbenzene sulfate: 0.5, sodium dodecylbenzene sulfate: 0.5 U potassium ascorbate sulfate ester: 0.5, sodium ascorbate sulfate ester: 0.5 e potassium sulfate: 0.0005, sodium sulfate: 0.0005 f potassium sulfate: 1.5, sodium sulfate: 1.5

Embodiment 1-1

The inner surfaces of the cavities of the pressing dies were previously maintained at a temperature of 150° C. Then, the lubricants A to G, a and b shown in Table 1-1 were applied to the inner surfaces of the cavities at a different thickness, and then water was evaporated. Thereby, crystallization layers having a different thickness were formed on the inner surfaces of the cavities, which consisted of potassium sulfate, potassium sulfite, potassium thiosulfate, potassium dodecyl sulfate, potassium dodecylbenzene sulfate, food blue No. 1 and potassium ascorbate sulfate ester.

Meanwhile, into the cavities of the pressing dies formed with the crystallization layers were filled the mixed powders that were obtained by blending the raw powders of Fe-1.5% Mo iron base alloy powder and C powder to be an ingredient composition of Fe-1.5% Mo-0.3% C and mixing the blend with the V-shaped blender, and then the filled powders were pressed at a pressure of 800 MPa. Thereby, powder compacts having a high concentration of potassium (K) layer on their surfaces were formed. These obtained powder compacts having the high concentration of potassium (K) layer on their surfaces were sintered under a gas atmosphere of 5% H₂—N₂ at a temperature of 1150° C., and further subjected to carburizing quenching and tempering. Thereby, iron base sintered alloys 1 to 7 of the present invention and comparative iron base sintered alloys 1 to 4 were produced, which contained potassium (K) in surface layer portions thereof.

CONVENTIONAL EXAMPLE 1-1

A lubricant that dispersed lithium stearate powder having an average particle size of 5 μm into acetone was applied to the inner surface of the cavity of the pressing die that was heated at 150° C. and prepared in Embodiment 1, and then acetone was evaporated. Thereby, a layer of lithium stearic sulfate was formed on the inner surface of the cavity of the pressing die. Then, the iron base alloy mixed powder that had the ingredient composition of Fe-1.5% Mo-0.3% C and was prepared in Embodiment 1-1 was filled into the cavity of the pressing die, and was pressed at 800 MPa, thereby being formed into a powder compact. The powder compact was drawn out and sintered under a gas atmosphere of 5% H₂—N₂ at a temperature of 1150° C. Thereby, a conventional iron base sintered alloy 1 was produced.

In the iron base sintered alloys 1 to 7 of the invention, the comparative iron base sintered alloys 1 to 4, and the conventional iron base sintered alloy 1, thickness and average concentration of K in the surface layer portion thereof were measured by EPMA (Electron Probe Microanalysis) The measured results were shown in Table 1-2. Further, Rockwell hardness, open porosity and alloy's density in the surfaces of these alloys were measured. The measured results were shown in Table 1-2. TABLE 1-2 Fe base sintered alloy surface layer portion Rockwell Fe base Average conc. of hardness Open Density sintered Lubricant Thickness K in surface layer of surface porosity of alloy alloy in Table 1 (mm) portion (mass %) layer (HRA) (%) (Mg/m³) Invention 1 A 0.003 0.0003 83 4.9 7.4 2 B 0.07 0.06 84 2.5 7.4 3 C 0.3 0.3 86 0.8 7.41 4 D 0.5 0.5 86 0.6 7.4 5 E 1.2 1.2 84 0.3 7.39 6 F 1.8 1.8 83 0.1 7.41 7 G 0.15 0.15 86 1.0 7.4 Comparative 1 a 1 Less than 0.0001* 76 7.0 7.41 examples 2 b 1 1.2* 70 0.9 7.4 3 A 0.0005* 0.5 75 6.5 7.39 4 A 2.2* 0.5 71 1.3 7.41 Related art 1 75 7.1 7.41 *indicates values outside the range of the invention.

It could be seen from the results shown in Table 1-2 that the iron base sintered alloys 1 to 7 of the invention which were produced by use of the lubricant solving the potassium sulfate salt into the solvent were substantially same in density as compared with the conventional iron base sintered alloy 1 which was produced by use of the lubricant dispersing the lithium stearate powder into acetone, but that, because the iron base sintered alloys 1 to 7 of the invention all were high in surface hardness and were also remarkably reduced in surface open porosity, they had dense surface and excellent surface wear resistance. However, it could be seen from the results shown in Table 1-2 that some of the comparative iron base sintered alloys 1 to 4 having a value beyond a range of the invention showed an unfavorable characteristic.

Embodiment 1-2

The inner surfaces of the cavities of the pressing dies were previously maintained at a temperature of 150° C. Then, the lubricants H to N, c and d shown in Table 1-1 were applied to the inner surfaces of the cavities at a different thickness, and then water was evaporated. Thereby, crystallization layers having a different thickness were formed on the inner surfaces of the cavities, which consisted of disodium hydrogen sulfate, trisodium sulfate, polysodium sulfate, and sodium riboflavin sulfate.

Meanwhile, into the cavities of the pressing dies formed with the crystallization layers were filled the mixed powders obtained by blending the raw powders of Fe-1.5% Mo iron base alloy powder and C powder to be an ingredient composition of Fe-1.5% Mo-0.3% C and mixing the blend with the V-shaped blender, and then the filled powders were pressed at a pressure of 800 MPa. Thereby, powder compacts having a high concentration of sodium (Na) layer on their surfaces were formed. These powder compacts having the high concentration of sodium (Na) layer on their surfaces were sintered under a gas atmosphere of 5% H₂-N₂ at a temperature of 1150° C., and further subjected to carburizing quenching and tempering. Thereby, iron base sintered alloys 8 to 14 of the invention and comparative iron base sintered alloys 5 to 8 were produced, which were shown in Table 1-3 and had an average concentration of Na in iron base sintered alloy surface layer portion.

In the iron base sintered alloys 8 to 14 of the invention and the comparative iron base sintered alloys 5 to 8, thickness and average concentration of Na in the surface layer portion thereof were measured by EPMA. The measured results were shown in Table 1-3. Further, Rockwell hardness, open porosity and alloy's density in the surfaces of these alloys were measured. The measured results were shown in Table 1-3. TABLE 1-3 Fe base sintered alloy surface layer portion Rockwell Fe base Average conc. of hardness Open Density sintered Lubricant Thickness K in surface layer of surface porosity of alloy alloy in Table 1 (mm) portion (mass %) layer (HRA) (%) (Mg/m³) Invention 8 H 0.003 0.003 83 4.2 7.4 9 I 0.07 0.06 84 1.8 7.42 10 J 0.3 0.3 86 0.8 7.39 11 K 0.5 0.5 87 0.4 7.39 12 L 1.2 1.2 85 0.2 7.39 13 M 1.8 1.8 84 0.1 7.4 14 N 0.15 0.15 86 0.9 7.41 Comparative 5 c 1 Less than 0.0001* 75 7.4 7.39 examples 6 d 1 1.2* 72 0.6 7.39 7 H 0.0005* 0.5 74 6.6 7.4 8 H 2.2* 0.5 70 0.5 7.39 *indicates values outside the range of the invention.

It could be seen from the results shown in Table 1-3 that the iron base sintered alloys 8 to 14 of the invention which were produced by use of the lubricant solving the potassium sulfate salt into the solvent were substantially same in density as compared with the conventional iron base sintered alloy 1 of Table 1-1 which was produced by use of the lubricant dispersing the lithium stearate powder into acetone, but that, because the iron base sintered alloys 8 to 14 of the invention all were high in surface hardness and were also remarkably reduced in surface open porosity, they had dense surface and excellent surface wear resistance. However, it could be seen from the results shown in Table 1-3 that some of the comparative iron base sintered alloys 5 to 8 having a value beyond a range of the invention showed an unfavorable characteristic.

Embodiment 1-3

The inner surfaces of the cavities of the pressing dies were previously maintained at a temperature of 150° C. Then, the lubricants O to U, e and f shown in Table 1-1 were applied to the inner surfaces of the cavities at a different thickness, and then water was evaporated. Thereby, crystallization layers were formed on the inner surfaces of the cavities, which consisted of a mixed salt of a potassium sulfate salt and a sodium sulfate salt.

Meanwhile, into the cavities of the pressing dies formed with the crystallization layers were filled the mixed powders that were obtained by blending the raw powders of Fe-1.5% Mo iron base alloy powder and C powder to be an ingredient composition of Fe-1.5% Mo-0.3% C and mixing the blend with the V-shaped blender, and then the filled powders were pressed at a pressure of 800 MPa. Thereby, powder compacts having a high concentration of potassium (K) and sodium (Na) layer on their surfaces were formed. These obtained powder compacts having the high concentration of K and Na layer on their surfaces were sintered under a gas atmosphere of 5% H₂—N₂ at a temperature of 1150° C., and further subjected to carburizing quenching and tempering. Thereby, iron base sintered alloys 15 to 21 of the invention and comparative iron base sintered alloys 9 to 12 were produced, which were high in K and Na concentrations in iron base sintered alloy surface layer portions.

In the iron base sintered alloys 15 to 21 of the invention and the comparative iron base sintered alloys 9 to 12, thickness, average concentration of K and average concentration of Na in the iron base sintered alloy surface layer portion were measured by EPMA. The measured results were shown in Table 1-4. Further, Rockwell hardness, open porosity and alloy's density in the surfaces of these alloys were measured. The measured results were shown in Table 1-4. TABLE 1-4 Fe base sintered alloy surface layer portion Rockwell Fe base Average concentration in hardness Open Density sintered Lubricant Thickness surface layer portion (mass %) of surface porosity of alloy alloy in Table 1 (mm) K Na layer (HRA) (%) (Mg/m³) Invention 15 O 0.003 0.0015 0.0015 83 7.4 7.41 16 P 0.07 0.03 0.03 85 1.9 7.41 17 Q 0.3 0.15 0.15 86 0.4 7.38 18 R 0.5 0.25 0.25 87 0.3 7.39 19 S 1.2 0.3 0.3 84 0.2 7.4 20 T 1.8 0.45 0.45 83 0.1 7.41 21 U 0.9 0.075 0.075 87 1.3 7.42 Comparative 9 e 1 Less than 0.0001* Less than 0.0001* 75 7.2 7.4 examples 10 f 1 0.6* 0.6* 72 0.2 7.4 11 O 0.0005* 0.25 0.25 74 6.8 7.4 12 O 2.2* 0.25 0.25 71 0.1 7.41 *indicates values outside the range of the invention.

It could be seen from the results shown in Table 1-4 that the iron base sintered alloys 15 to 21 of the invention which were produced by use of the lubricant solving the mixed salt of the potassium sulfate salt and sodium sulfate salt into the solvent were substantially same in density as compared with the conventional iron base sintered alloy 1 of Table 1-1 which was produced by use of the lubricant dispersing the lithium stearate powder into acetone, but that, because the iron base sintered alloys 15 to 21 of the invention all were high in surface hardness and were also remarkably reduced in surface open porosity, they had dense surface and excellent surface wear resistance. However, it could be seen from the results shown in Table 1-4 that some of the comparative iron base sintered alloys 9 to 12 having a value beyond a range of the invention showed an unfavorable characteristic.

Embodiment 2

Pressing dies were prepared, each of which had a cavity of a diameter of 11 mm and a height of 40 mm, and inner surfaces of the cavities of which could be heated at a predetermined temperature. Further, as raw powders, Fe-1.5% Mo iron base alloy powder having an average particle size of 90 μm and C powder of an average particle size of 20 μm were prepared, and blended and mixed to be Fe-1.5% Mo-0.3% C with a V-shaped blender. Thereby, mixed powders were obtained. A content of phosphor (P) which was contained in the mixed powders was 0.01%.

Further, there were prepared dipotassium hydrogen phosphate, tripotassium phosphate, polypotassium phosphate, potassium riboflavin phosphate, disodium hydrogen phosphate, trisodium phosphate, polysodium phosphate, sodium riboflavin phosphate, and these salts were solved into a solvent, water, in a different concentration. Thereby, lubricants A to L and a to f denoted in Table 2-1 were obtained. TABLE 2-1 Composition (Mass %) Type Additives Balance Lubricant A dipotassium hydrogen phosphate: 0.1 water B tripotassium phosphate: 0.5 C polypotassium phosphate: 1 D potassium riboflavin phosphate: 10 a dipotassium hydrogen phosphate: 0.001 b tripotassium phosphate: 20 E disodium hydrogen phosphate: 0.1 F trisodium phosphate: 0.5 G polysodium phosphate: 1 H sodium riboflavin phosphate: 10 c disodium hydrogen phosphate: 0.001 d trisodium phosphate: 30 I dipotassium hydrogen phosphate: 0.3, trisodium phosphate: 0.3 J tripotassium phosphate: 0.5, sodium riboflavin phosphate: 0.5 K potassium riboflavin phosphate: 1, polysodium phosphate: 1 L polypotassium phosphate: 3, disodium hydrogen phosphate: 3 e dipotassium hydrogen phosphate: 0.0005, trisodium phosphate: 0.0005 f dipotassium hydrogen phosphate: 15, trisodium phosphate: 15

Embodiment 2-1

The inner surfaces of the cavities of the pressing dies were previously maintained at a temperature of 150° C. Then, the lubricants A to D, a and b shown in Table 2-1 were applied to the inner surfaces of the cavities at a different thickness, and then water was evaporated. Thereby, crystallization layers having a different thickness were formed on the inner surfaces of the cavities, which consisted of dipotassium hydrogen phosphate, tripotassium phosphate, polypotassium phosphate, potassium riboflavin phosphate.

Meanwhile, into the cavities of the pressing dies formed with the crystallization layers were filled the mixed powders that were obtained by blending the raw powders of Fe-1.5% Mo iron base alloy powder and C powder to be an ingredient composition of Fe-1.5% Mo-0.3% C and mixing the blend with the V-shaped blender, and then the filled powders were pressed at a pressure of 800 MPa. Thereby, powder compacts having a high concentration of phosphor (P) and potassium (K) layer on their surfaces were formed. These obtained powder compacts having the high concentration of P and K layer on their surfaces were sintered under a gas atmosphere of 5% H₂—N₂ at a temperature of 1150° C., and further subjected to carburizing quenching and tempering. Thereby, iron base sintered alloys 1 to 4 of the invention and comparative iron base sintered alloys 1 to 4 were produced, which were high in P and K concentrations in the iron base sintered alloy surface layer portions.

CONVENTIONAL EXAMPLE 2-1

A lubricant that dispersed lithium stearate powder having an average particle size of 5 μm into acetone was applied to the inner surface of the cavity of the pressing die that was heated at 150° C. and prepared in Embodiment 2-1, and then acetone was evaporated. Thereby, a layer of lithium stearate was formed on the inner surface of the cavity of the pressing die. Then, the iron base alloy mixed powder that had the ingredient composition of Fe-1.5% Mo-0.3% C and was prepared in Embodiment 2-1 was filled into the cavity of the pressing die, and was pressed at 800 MPa, thereby being formed into a powder compact. The powder compact was drawn out and sintered under a gas atmosphere of 5% H₂-N₂ at a temperature of 1150° C. Thereby, a conventional iron base sintered alloy 1 was produced.

In the iron base sintered alloys 1 to 4 of the invention, the comparative iron base sintered alloys 1 to 4, and the conventional iron base sintered alloy 1, an average concentration of P and an average concentration of K in the iron base sintered alloy surface layer portions of the iron base sintered alloys, as well as a concentration difference of P that extracted a concentration of P in the iron base sintered alloy inner portions from the average concentration of P were measured, and the measured results were shown in Table 2-2. Further, Rockwell hardness, open porosity and alloy's density in the surfaces of the iron base sintered alloys were measured, and the measured results were shown in Table 2-2. TABLE 2-2 Fe base sintered alloy surface layer portion Average conc. Average conc. Average conc. Rockwell Fe base of P in surface of P - conc. of K in surface hardness Open Density sintered Lubricant layer portion of inside P layer portion Thickness of surface porosity of alloy alloy in Table 1 (mass %) (mass %) (mass %) (mm) layer (HRA) (%) (Mg/m³) Invention 1 A 0.03 0.02 0.0002 0.03 81 4.8 7.39 2 B 0.14 0.13 0.021 0.6 83 0.9 7.39 3 C 0.33 0.32 0.053 1.3 85 0.6 7.4 4 D 0.95 0.94 0.47 1.8 86 0.2 7.4 Comparative 1 a 0.015 0.005* Less than 0.0001* 0.002 74 6.5 7.4 examples 2 b 1.2 1.19* 1.2* 1.7 67 0.2 7.41 3 A 0.03 0.02 0.011 0.0005* 75 7.3 7.39 4 A 0.85 0.84 0.92 2.2* 69 0.1 7.4 Related art 1 75 7.1 7.41 *indicates values outside the range of the invention.

It could be seen from the results shown in Table 2-2 that the iron base sintered alloys 1 to 4 of the invention which were produced by use of the lubricant solving the metallic salt of potassium phosphate into the solvent were substantially same in density as compared with the conventional iron base sintered alloy 1 which was produced by use of the lubricant dispersing the lithium stearate powder into acetone, but that, because the iron base sintered alloys 1 to 4 of the invention all were high in surface hardness and were also remarkably reduced in surface open porosity, they had dense surface and excellent surface wear resistance. However, it could be seen from the results shown in Table 2-2 that some of the comparative iron base sintered alloys 1 to 4 having a value beyond a range of the invention showed an unfavorable characteristic.

Further, with respect to each of the iron base sintered alloys 1 to 4 of the invention and the comparative iron base sintered alloys 1 to 4, contents of P and K from its surface toward its center were measured by EPMA. As shown in Table 2-2, it could be seen that an average value of the P concentrations of the iron base sintered alloy surface layer portions was higher than the P concentration in the iron base sintered alloy inner portions, and K was also contained in the iron base sintered alloy surface layer portions.

Embodiment 2-2

The inner surfaces of the cavities of the pressing dies were previously maintained at a temperature of 150° C. Then, the lubricants E to H, c and d shown in Table 2-1 were applied to the inner surfaces of the cavities at a different thickness, and then water was evaporated. Thereby, crystallization layers having a different thickness were formed on the inner surfaces of the cavities, which consisted of disodium hydrogen phosphate, trisodium phosphate, polysodium phosphate, sodium riboflavin phosphate.

Meanwhile, into the cavities of the pressing dies formed with the crystallization layers were filled the mixed powders that were obtained by blending the raw powders of Fe-1.5% Mo iron base alloy powder and C powder to be an ingredient composition of Fe-1.5% Mo-0.3% C and mixing the blend with the V-shaped blender, and then the filled powders were pressed at a pressure of 800 MPa. Thereby, powder compacts having a high concentration of phosphor (P) and sodium (Na) layer on their surfaces were formed. These obtained powder compacts having the high concentration of P and Na layer on their surfaces were sintered under a gas atmosphere of 5% H₂—N₂ at a temperature of 1150° C., and further subjected to carburizing quenching and tempering. Thereby, iron base sintered alloys 5 to 8 of the invention and comparative iron base sintered alloys 5 to 8 were produced, which were high in P and Na concentrations in the iron base sintered alloy surface layer portions.

In the iron base sintered alloys 5 to 8 of the invention and the comparative iron base sintered alloys 5 to 8, a thickness, an average concentration of P and an average concentration of Na in the iron base sintered alloy surface layer portions of the iron base sintered alloys, as well as a concentration difference of P that extracted a concentration of P in the iron base sintered alloy inner portions from the average concentration of P were measured, and the measured results were shown in Table 2-3. Further, Rockwell hardness, open porosity and alloy's density in the surfaces of the iron base sintered alloys were measured, and the measured results were shown in Table 2-3. TABLE 2-3 Fe base sintered alloy surface layer portion Average conc. Average conc. Average conc. Rockwell Fe base of P in surface of P - conc. of K in surface hardness Open Density sintered Lubricant layer portion of inside P layer portion Thickness of surface porosity of alloy alloy in Table 1 (mass %) (mass %) (mass %) (mm) layer (HRA) (%) (Mg/m³) Invention 5 E 0.05 0.04 0.0006 1.2 86 3.1 7.41 6 F 0.21 0.20 0.054 1.8 87 0.9 7.4 7 G 0.43 0.42 0.29 0.15 85 0.4 7.41 8 H 0.96 0.95 0.94 0.42 85 0.1 7.39 Comparative 5 c 0.015 0.0005* Less than 0.0001 0.002 74 6.5 7.4 examples 6 d 1.2 1.19* 1.4 1.7 67 0.2 7.41 7 D 0.03 0.02 0.002 0.0005* 75 7.3 7.39 8 D 0.85 0.84 0.85 2.2* 69 0.1 7.4 *indicates values outside the range of the invention

It could be seen from the results shown in Table 2-3 that the iron base sintered alloys 5 to 8 of the invention which were produced by use of the lubricant solving the metallic salt of potassium phosphate into the solvent were substantially same in density as compared with the conventional iron base sintered alloy 1 of Table 2-1 which was produced by use of the lubricant suspending the lithium stearate powder into acetone, but that, because the iron base sintered alloys 5 to 8 of the invention all were high in surface hardness and were also remarkably reduced in surface open porosity, they had dense surface and excellent surface wear resistance. However, it could be seen from the results shown in Table 2-3 that some of the comparative iron base sintered alloys 5 to 8 having a value beyond a range of the invention showed an unfavorable characteristic.

Further, with respect to each of the iron base sintered alloys 5 to 8 of the invention and the comparative iron base sintered alloys 5 to 8, contents of P and K from its surface toward its center were measured by EPMA. As shown in Table 2-3, it could be seen that an average value of the P concentrations of the iron base sintered alloy surface layer portions was higher than the P concentration in the iron base sintered alloy inner portions, and Na was also contained in the iron base sintered alloy surface layer portions.

Embodiment 2-3

The inner surfaces of the cavities of the pressing dies were previously maintained at a temperature of 150° C. Then, the lubricants I to L, e and f shown in Table 2-1 were applied to the inner surfaces of the cavities at a different thickness, and then water was evaporated. Thereby, crystallization layers were formed on the inner surfaces of the cavities, which consisted of a mixed salt of a potassium phosphate metallic salt and a sodium phosphate metallic salt.

Meanwhile, into the cavities of the pressing dies formed with the crystallization layers were filled the mixed powders that were obtained by blending the raw powders of Fe-1.5% Mo iron base alloy powder and C powder to be an ingredient composition of Fe-1.5% Mo-0.3% C and mixing the blend with the V-shaped blender, and then the filled powders were pressed at a pressure of 800 MPa. Thereby, powder compacts having a high concentration of phosphor (P), potassium (K) and sodium (Na) layer on their surfaces were formed. These obtained powder compacts having the high concentration of P, K and Na layer on their surfaces were sintered under a gas atmosphere of 5% H₂—N₂ at a temperature of 1150° C., and further subjected to carburizing quenching and tempering. Thereby, iron base sintered alloys 9 to 12 of the invention and comparative iron base sintered alloys 9 to 12 were produced, which were high in P, K and Na concentrations in the iron base sintered alloy surface layer portions.

In the iron base sintered alloys 9 to 12 of the invention and the comparative iron base sintered alloys 9 to 12, an average concentration of P in the iron base sintered alloy surface layer portions thereof and a concentration difference of P that extracted a concentration of P in the iron base sintered alloy inner portions from the average concentration of P in the iron base sintered alloy surface layer portions were measured. Further, an average concentration of K and an average concentration of Na in the iron base sintered alloy surface layer portions were measured. The measured results were shown in Table 2-4. Further, Rockwell hardness, open porosity and alloy's density in the surfaces of the iron base sintered alloys were measured, and the measured results were shown in Table 2-4. TABLE 2-4 Fe base sintered alloy surface layer portion Average conc. Average conc. Rockwell Fe base of P in surface of P - conc. Average concentration hardness Open Density sintered Lubricant layer portion of inside P (mass %) Thickness of surface porosity of alloy alloy in Table 1 (mass %) (mass %) K Na (mm) layer (HRA) (%) (Mg/m³) Invention 9 I 0.15 0.14 0.014 0.011 1.2 86 0.5 7.41 10 J 0.35 0.34 0.035 0.042 1.8 87 0.4 7.4 11 K 0.49 0.48 0.21 0.18 0.15 85 0.2 7.41 12 L 0.75 0.74 0.45 0.47 0.42 85 0.1 7.39 Comparative 9 e 0.015 0.0005* Less than 0.0001* Less than 0.0001* 0.002 74 6.5 7.4 examples 10 f 1.2 1.19* 0.63* 0.63* 1.7 67 0.2 7.41 11 I 0.03 0.02 0.022 0.018 0.0005* 75 7.3 7.39 12 I 0.85 0.84 0.92* 0.94* 2.2* 69 0.1 7.4 *indicates values outside the range of the invention

It could be seen from the results shown in Table 2-4 that the iron base sintered alloys 9 to 12 of the invention produced by use of the lubricant solving the mixed salt of the potassium phosphate metallic salt and sodium phosphate metallic salt into the solvent were substantially same in density as compared with the conventional iron base sintered alloy 1 of Table 2-1 produced by use of the lubricant suspending the lithium stearate powder into acetone, but, because the iron base sintered alloys 9 to 12 of the invention all were high in surface hardness and were also remarkably reduced in surface open porosity, they had dense surface and excellent surface wear resistance. However, it could be seen from the results shown in Table 2-4 that some of the comparative iron base sintered alloys 9 to 12 having a value beyond a range of the invention showed an unfavorable characteristic.

Further, with respect to each of the iron base sintered alloys 9 to 12 of the invention and the comparative iron base sintered alloys 9 to 12, contents of P, K and Na from its surface toward its center were measured by EPMA. As shown in Table 2-4, it could be seen that an average value of the P concentrations of the iron base sintered alloy surface layer portions was higher than the P concentration in the iron base sintered alloy inner portions, and K and Na were also contained in the iron base sintered alloy surface layer portions. 

1. An iron base sintered alloy having highly densified and hardened surface, comprising: a surface layer portion of the iron base sintered alloy having a thickness in a range of 1 μm to 2 mm from a surface of the iron base sintered alloy; and an inner portion of the iron base sintered alloy located inside the iron base sintered alloy surface layer portion, wherein the iron base sintered alloy surface layer portion contains potassium K.
 2. An iron base sintered alloy having highly densified and hardened surface, comprising: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, wherein the iron base sintered alloy surface layer portion contains sodium Na.
 3. An iron base sintered alloy having highly densified and hardened surface, comprising: an iron base sintered alloy surface layer portion; and an iron base sintered alloy inner portion, wherein the iron base sintered alloy surface layer portion contains K and Na.
 4. The iron base sintered alloy according to claim 1, wherein the iron base sintered alloy surface layer portion contains K in an average concentration of 0.0001 to 1 mass %.
 5. The iron base sintered alloy according to claim 2, wherein the iron base sintered alloy surface layer portion contains Na in an average concentration of 0.0001 to 1 mass %.
 6. The iron base sintered alloy according to claim 3, wherein the iron base sintered alloy surface layer portion contains K and Na in an average concentration of 0.0001 to 1 mass % in total.
 7. The iron base sintered alloy according to claim 1, wherein the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, K: 0.0001 to 1 mass % in the average concentration, and balance: Fe and unavoidable impurities.
 8. The iron base sintered alloy according to claim 2, wherein the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, Na: 0.0001 to 1 mass % in the average concentration, and balance: Fe and unavoidable impurities.
 9. The iron base sintered alloy according to claim 3, wherein the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, K and Na: 0.0001 to 1 mass % in the average concentration in total, and balance: Fe and unavoidable impurities.
 10. The iron base sintered alloy according to claim 1, wherein the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, K: 0.0001 to 1 mass % in the average concentration, and balance: Fe and unavoidable impurities.
 11. The iron base sintered alloy according to claim 2, wherein the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, Na: 0.0001 to 1 mass % in the average concentration, and balance: Fe and unavoidable impurities.
 12. The iron base sintered alloy according to claim 3, wherein the iron base sintered alloy inner portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, and balance: Fe and unavoidable impurities; and the iron base sintered alloy surface layer portion has a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, K and Na : 0.0001 to 1 mass % in the average concentration in total, and balance: Fe and unavoidable impurities.
 13. The iron base sintered alloy according to claim 1, wherein the iron base sintered alloy surface layer portion has an open porosity of 5% or less.
 14. A mechanical part made of the iron base sintered alloy according to claim
 1. 15. The mechanical part according to claim 14, wherein the mechanical part is a sprocket, a gear, a rotor, an inner race or an outer race. 16-19. (canceled)
 20. An iron base sintered alloy having highly densified and hardened surface, comprising: a surface layer portion of the iron base sintered alloy having a thickness in a range of 1 μm to 2 mm from a surface of the iron base sintered alloy, said surface layer having an average concentration of phosphor P, higher than a concentration of P contained in an inner portion of the iron base sintered alloy located inside the iron base sintered alloy surface layer portion, and the iron base sintered alloy surface layer portion contains potassium K.
 21. An iron base sintered alloy having highly densified and hardened surface, comprising: an iron base sintered alloy surface layer portion having an average concentration of P higher than a concentration of P contained in an iron base sintered alloy inner portion, and the iron base sintered alloy surface layer portion containing sodium Na.
 22. An iron base sintered alloy having highly densified and hardened surface, comprising: an iron base sintered alloy surface layer portion having an average concentration of P higher than a concentration of P contained in an iron base sintered alloy inner portion, and the iron base sintered alloy surface layer portion containing K and Na.
 23. The iron base sintered alloy according to claim 20, wherein the average concentration of P of the iron base sintered alloy surface layer portion is higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass %, and the iron base sintered alloy surface layer portion contains K in an average concentration of 0.0001 to 1 mass %.
 24. The iron base sintered alloy according to claim 21, wherein the average concentration of P of the iron base sintered alloy surface layer portion is higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass %, and the iron base sintered alloy surface layer portion contains Na in an average concentration of 0.0001 to 1 mass %.
 25. The iron base sintered alloy according to claim 22, wherein the average concentration of P of the iron base sintered alloy surface layer portion is higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass %, and the iron base sintered alloy surface layer portion contains K and Na in an average concentration of 0.0001 to 1 mass % in total.
 26. The iron base sintered alloy according to claim 20, wherein the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains K in an average concentration of 0.0001 to 1 mass %.
 27. The iron base sintered alloy according to claim 21, wherein the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains Na in an average concentration of 0.0001 to 1 mass %.
 28. The iron base sintered alloy according to claim 22, wherein the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains K and Na in an average concentration of 0.0001 to 1 mass % in total.
 29. The iron base sintered alloy according to claim 20, wherein the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains K in an average concentration of 0.0001 to 1 mass %.
 30. The iron base sintered alloy according to claim 21, wherein the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to 1 mass % and contains Na in an average concentration of 0.0001 to 1 mass %.
 31. The iron base sintered alloy according to claim 22, wherein the iron base sintered alloy inner portion is made up of an iron base alloy sintered body having a composition of C: 0.1 to 0.9 mass %, one or at least two of Mo, Ni, Cr and Cu: 0.1 to 10 mass % in total, one or at least two of Mn, Si, V, Nb, W, Al, Ti, Ca and S: 0.1 to 3 mass % in total, P: 0.05 mass % or less (including 0 mass %), and balance: Fe and unavoidable impurities, and the iron base sintered alloy surface layer portion has the average concentration of P higher than the concentration of P contained in the iron base sintered alloy inner portion by 0.01 to I mass % and contains K and Na in an average concentration of 0.0001 to 1 mass % in total.
 32. The iron base sintered alloy according to claim 20, wherein the iron base sintered alloy surface layer portion has open porosity of 5% or less.
 33. A mechanical part made of the iron base sintered alloy according to claim
 20. 34. The mechanical part according to claim 33, wherein the mechanical part is a sprocket, a gear, a rotor, an inner race, or an outer race. 35-38. (canceled)
 39. A producing method of the iron base sintered alloy according to claim 1, comprising: forming a coat of a potassium sulfate salt consisting of one or at least two of potassium sulfate, potassium sulfite, potassium thiosulfate, potassium dodecyl sulfate, potassium dodecylbenzene sulfate, food blue No. 1 and potassium ascorbate sulfate ester, acting as a lubricant, on an inner surface of a cavity of a pressing die; filling raw powder of the iron base sintered alloy into the cavity of the pressing die and then pressing the filled raw powder; and sintering the formed powder compact.
 40. A producing method of the iron base sintered alloy according to claim 2, comprising: forming a coat of a sodium sulfate salt consisting of one or at least two of sodium sulfate, sodium sulfite, sodium thiosulfate, sodium dodecyl sulfate, sodium dodecylbenzene sulfate, food blue No. 5 and sodium ascorbate sulfate ester, acting as a lubricant, on an inner surface of a cavity of a pressing die; filling raw powder of the iron base sintered alloy into the cavity of the pressing die and then pressing the filled raw powder; and sintering the formed powder compact.
 41. A producing method of the iron base sintered alloy according to claim 3, comprising: forming a mixed coat of a potassium sulfate salt and a sodium sulfate salt, acting as a lubricant, on an inner surface of a cavity of a pressing die; filling raw powder of the iron base sintered alloy into the cavity of the pressing die and then pressing the filled raw powder; and sintering the formed powder compact.
 42. A producing method of the iron base sintered alloy according to claim 20, comprising: forming a coat of the potassium phosphate metallic salt consisting of one or at least two of dipotassium hydrogen phosphate, tripotassium phosphate, polypotassium phosphate, potassium riboflavin phosphate, acting as a lubricant, on an inner surface of a cavity of a pressing die; filling raw powder of the iron base sintered alloy into the cavity of the pressing die and then pressing the filled raw powder; and sintering the formed powder compact.
 43. A producing method of the iron base sintered alloy according to claim 21, comprising: forming a coat of the sodium phosphate metallic salt consisting of one or at least two of disodium hydrogen phosphate, trisodium phosphate, polysodium phosphate, sodium riboflavin phosphate, acting as a lubricant, on an inner surface of a cavity of a pressing die; filling raw powder of the iron base sintered alloy into the cavity of the pressing die and then pressing the filled raw powder; and sintering the formed powder compact.
 44. A producing method of the iron base sintered alloy according to claim 22, comprising: forming a coat of the mixed metallic salt of the potassium phosphate metallic salt and the sodium phosphate metallic salt, acting as a lubricant, on an inner surface of a cavity of a pressing die; filling raw powder of the iron base sintered alloy into the cavity of the pressing die and then pressing the filled raw powder; and sintering the formed powder compact. 